Beginning in the 1960's, foamed thermoplastic containers began to compete with containers traditionally made of paper pulp or comparatively dense organic plastic materials. U.S. Pat. Nos. 3,845,187 and 3,862,817, the disclosures of which are incorporated herein by reference in their entirety, are concerned with the thermoplastic molding of containers such as egg cartons and the like.
U.S. Pat. No. 3,845,187 emphasizes at col. 1, lines 64-66, the disadvantages involved in the then-current technique of first molding a container and in a second downstream operation employing hole-punching equipment to form latch openings in the container. U.S. Pat. No. 3,845,187 discloses at a col. 1, lines 66-70 "a way of forming the desired latch opening during the molding process itself with a relatively simple and workable variation in the mold to eliminate the entire hole-punching accessory and the problem it causes."
The aforementioned patents teach that latch hole openings can be formed in the top front cover of egg cartons during the process of molding the container. They teach that moldable plastic is shape molded by forming a compression between a pair of cooperating mold members; and, during this molding, key members cooperate with a combination of a bevel and oblique surfaces to first pierce the plastic, and second, continuation of relative motion between the key members, enlarges the regions pierced to create the needed orifices by scraping and tearing the side edges. These orifices then serve as latch openings. They cooperate with a lower flap member and together form a secure latching arrangement which can keep the container structure closed.
Automatic thermoformation equipment designed to form a plurality of structures simultaneously are normally massive pieces of equipment which are expected to continuously operate up to three work shifts per day, seven days a week. This equipment utilizes male and female mold members which define the thermoformed structures. Each half of the mold is a dense machined tool member which must stand up to repetitive thermoformation operations which not only involve physical force but rapid and wide changes in temperature. Failure of even the smallest component of the mold structures will cause a shutdown of the thermoformation apparatus.
One cause of system shutdown involves the key members utilized in the mold members to form orifices in the containers. In order to extend the life of such key members, the mold members can be made to provide precise key member registration or, alternatively, at least one of the key members can be made to yield in a direction transverse to the closure of the molds in order to avoid destruction or early wear-related failure. In the latter case, these key members are usually spring biased so that this transverse motion can be permitted with return of the key member to its proper location after the transverse motion is completed. It has been suggested to either place a resilient member or a spring member behind one or the other of the key members so that during mold closures any misalignment of the key member will not lead to early self-destruction. These spring or elastic means have not always served the purpose for a reasonable period of time. Failure of the resilient or spring member leads to failure of the key members soon thereafter, or inadequate formation of latch holes.
Therefore, it is an object of the present invention to improve the reliability of the members employed to form the desired orifices in plastic parts.